Conventionally, an ethylene polymer, a propylene polymer, a polymer of ethylene or propylene and an other olefin, or the like has been generally broadly used as resin for food containers.
Recently, so-called boilable foods capable of being warmed in boiling water in a container in which the foods are packed have been particularly popularized, and the foregoing resin has been used for containers for such retortable foods.
Containers for such materials such as foods are printed in a predetermined manner. For example, container of a final shape or sheets to be formed into the containers are prepared and then subjected to a print coating treatment so that names and features of foods are specified and/or colors or patterns are applied to improve the aesthetics of the containers.
However, since olefin polymers to be used for the printed containers for retortable materials are non-polar and crystalline, they generally are chemically inert. Moreover, the solubility thereof against various solvents is low because of their high crystallinity. Consequently, printed containers for retortable materials utilizing such olefin polymers have poor retort-resistant property, that is, the affinity between printing ink and the polymer is so poor that the property of printing and coating on container surfaces is low and the adhesion of printed characters or patterns onto the container surface is weak. Accordingly, the printed characters and/or patterns tend to be easily separated from the container surface.
In order to improve the printing and coating properties as well as the print adhesion properties, a method has been proposed in which the polarity of the resin for forming containers is enhanced by adding a polymer or inorganic material having a polar group to the resin, and a method in which the affinity between printing ink and the resin is improved by a surface treatment such as corona discharge treatment, flame treatment or the like are known.
In conventional printed containers for retortable materials of the foregoing olefin polymers, however, it has not been possible even with the the above methods to sufficiently improve the printing and coating properties of the container surface. Moreover, it has been difficult to obtain excellent print adhesion properties in which printed characters and/or patterns do not separate from the container through the retort treatment. Therefore, at present, printed retort containers in which the retort-resistant property is excellent and on which printing can be performed are not commercially available.